$$3.1\times 1000+4.3\times 10^{3}$$
Multiply $3.1$ and $1000$ to get $3100$.$$3100+4.3\times 10^{3}$$
Calculate $10$ to the power of $3$ and get $1000$.$$3100+4.3\times 1000$$
Multiply $4.3$ and $1000$ to get $4300$.$$3100+4300$$
Add $3100$ and $4300$ to get $7400$.$$7400$$